This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
The exploration for and discovery of new oil reserves has become increasingly challenging and costly. Untapped reserves tend to be more difficult to identify and evaluate, and very often are located subsea, which further increases complexity and cost of the exploration and discovery of such reserves. Successful, efficient, and cost effective identification and evaluation of hydrocarbon-bearing reservoirs is therefore very desirable.
In exploration, seep detection has become an important tool to identify potential hydrocarbon resources in the subsurface. Oil and gas accumulations often leak hydrocarbons including methane, ethane, propane, butane, naphthalene, and benzene. These hydrocarbons may migrate toward the surface through a variety of pathways that result in seeps discharging hydrocarbons to the surface. These seeps are thus surface expressions of subsurface geological phenomena. In some instances, seeps may be laterally displaced at some distance away from the accumulation from which they originate. Seeps may be classified as macroseeps and microseeps, which differ in hydrocarbon volumes or areal extent over which the seep discharges.
Active hydrocarbon systems are often identified by sampling surface expressions of subsurface geological phenomena, such as faults or fracture zones, that are likely hydrocarbon conduits. Currently discharging seeps (“active” seeps) or paleo-seeps are typically identified by seismic survey interpretations and may also be located with ship-board, side-scan sonar, or other techniques during the survey. Usually only one core sample is taken at each feature. The core samples are usually several feet in length and are collected below the surface or below the water-sediment interface. The cores are then typically transported to land-based laboratories for analysis using fluorescence and standard petroleum geochemistry techniques. The costs of seep surveys are accordingly very high and may be in the range of a million US dollars for a forty sample survey. Due to the prohibitive costs, sampling density tends to be quite low. Accordingly there exists a need for identifying currently discharging seeps indicative of active hydrocarbon systems.
Once a likely site for the hydrocarbon accumulation has been established, an exploration well is drilled. Upon the drilling of the well, evaluation of the subsurface geology surrounding the well is typically achieved through indirect methods such as mud logging and well-based geophysical techniques such as electrical conductance, acoustics, and radioactive decay.
While formation evaluation techniques such as well logging remain the standard for the petroleum industry, these techniques are less than robust where challenging conditions exist. For instance, there may be cases where the presence of hydrocarbons, fluid type (gas, oil, and/or water), and proportion of hydrocarbon to water in the pore spaces are ambiguous even after formation evaluation. For example, carbonate reservoirs, thin-bedded clastic rocks, and wells containing very fresh water are particularly troublesome to evaluate using current standard techniques. Also, current formation evaluation techniques tend to be unable to distinguish moveable from immovable oil, particularly where the oil is biodegraded or severely altered. Contamination or invasion into the formation by hydrocarbon-based drilling fluids is yet another complication that makes distinguishing the natural hydrocarbon composition and quality using standard logging or geochemical methods much more difficult. Even further, when wells are drilled and only water is located in the potential reservoir unit, the standard formation evaluation techniques do not provide a reliable way to determine whether there are hydrocarbons in an updip or adjacent position (such as across a fault).
Accordingly there exists a substantial need for reliable, reproducible, efficient, robust, real-time and cost-effective means for identifying and evaluating hydrocarbon-bearing formations. In particular, there exists a substantial need for improving the efficacy and reliability of seep surveys, and to reduce the cost of seep surveys.